The invention concerns a down-draft fixed bed gasifier system, also known as a direct current solid bed gasification reactor, for the gasification of organic solids having high ash content and tending to form slag, in particular of residual or refuse materials such as sewage sludge, wood or liquid manure, with a solids storage chamber for the reception of the solid bordered at its bottom by a grate and with a product gas line for discharging the outgased product gases, wherein the solids storage chamber has an oxidation zone having an air and/or oxygen inlet. The invention is also directed to advantageous applications of the gasification reactor, in particular for the disposal of sewage sludge.
The sewage sludge generated in community or industrial sewage processing installations during the purification of sewage water is, in dried form, a solid having high caloric content of approximately 10,000-14,000 kJ/kg. When the sewage sludge is dried it solidifies and forms so-called pellets. These pellets can be burned in a gasification reactor or roasted to thereby give off a weak gas having high energy content (product gas) which can be burned in a diesel motor. A generator connected to this diesel motor is used to produce electricity. The waste heat of the motor can be utilized to dry the sewage sludge.
In accordance with prior art, the combustion of sewage sludge is only useful from an economic point of view if the sewage installation has a size corresponding to 0.5 million or 1 million serviced residents. The investment and operation costs do not result in an acceptable cost efficiency in smaller installations.
The conventional gasification reactors known to date can only be operated in batch mode, since the slag formed by same cannot be effectively and evenly discharged out of the combustion chamber through the shaker grate or via other discharge mechanisms. This means that the reactor is loaded with a charge of pellets and operated until all pellets are carbonized. The gasification procedure must subsequently be terminated and the slag removed from the combustion chamber.
By way of example, the gasification installation of the company IMBERT Energy Technology is configured as a suction gas installation having a solid fuel container with a solid storage chamber which tapers in a downward direction and is bordered at its lower end by a grating. An air conduit leading from the side into the solid storage chamber facilitates the introduction of air into the oxidation zone of the solids storage chamber. This solids storage chamber is disposed within a cylindrical reactor container having an exhaust opening in its upper region for suctioning-off the product gas. The IMBERT gasification installation works as a suction gas installation. An underpressure is applied to the exhaust opening integrated in the upper region of the reactor to suction the product gas out of the solids storage container. The product gas is thereby suctioned through the grate out of the solids storage container and guided past the outer wall of the solids storage container into the upper region of the reactor before it leaves the reactor.
These types of gasification reactors, or wood gas installations, are normally used to gasify wood or wood-like materials. These types of installations can also be used to gasify other materials, in particular dried sewage sludge shaped into pellets.
In order to gasify the sewage sludge, the sewage sludge pellets are roasted sub-stochiometrically. This is effected at a temperature between 850.degree. C. and 1100.degree. C. or even up to 1400.degree. C. The sewage sludge pellets form slag and a plurality of neighboring pellets combine into a large slag clump. This slag formation necessitates the emptying and cleaning of the gasification reactor in relatively short time intervals so that the gasification process must be interrupted. Each interruption has, however, the consequence that the gasification process must be renewed and the pellets must be brought up to operation temperature using externally input energy. For this reason, this type of gasification reactor has very low efficiency.
In addition, undesirable operation conditions occur in the heating up phase with regard to the gas quality from the materials initially roasting at low temperatures. An increased amount of tar and oil are present in the gas which deposit in the downstream conduits or can be improperly burned in the downstream motor to cause deposits.
Document DE-PS 425 634 discloses a flue gas generator operating in the counter current mode. This flue gas generator has a tapping blade in the vicinity of the decomposed slag for separating a portion of the slag which is subsequently transported out of the flue gas generator into a trolley or the like with the assistance of a pushing mechanism (push-out stamp).
This flue gas generator functions in accordance with the counter current principle. The device known in the art from document DE-PS 425634 cannot however be used for down-draft gasification since in down-draft gasification the product seats on a grating through which the gas is fed. The press-out stamp described above does not prevent residual slag from collecting in the intermediate grate spaces. This impedes processing and can lead to complete stoppage. The clogged grate can, as already described above, only be freed of the residual slag after termination of the gasification process so that this conventional device has the above described disadvantages with regard to economical use.